ECG Reference Notes

Standard ECG Calibration

•  Paper speed = 25 mm/s
• One large square = 0.2s
• One small square = 0.04s
• Amplitude = 10 mm/mV

Rate Calculation

• Assuming standard paper speed there are several methods to calculate the heart rate.
• Count number of large squares between consecutive R waves and divide into 300.
• Count number of small squares between consecutive R waves and divide into 1500.
• An ECG recoding captures10 seconds of activity, so you can multiple the number of complexes along the rhythm strip by 6 to estimate the rate, a useful method for irregular rhythms.

Normal Ranges

• PR Interval: 0.12 - 0.2s (3-5 small squares)
• QRS Duration: < 0.12s (3 small squares)
• QTc usually less than half the R-R interval.
• Consider prolonged QTc either > 440ms or > 460ms for males and >470ms for females.
• Considered shortened QTc if < 330ms

• QTc Calculators can be found here, here, and here

 Axis

• Normal: -30 degress to 90 degrees
• Left axis deviation: < -30 degrees
• Right axis deviation: > 90 degrees
• Extreme Axis: 180 degrees to -90 degrees

• Axis Calculator

Hex Axial

Image taken from http://en.wikipedia.org/wiki/Hexaxial_reference_system

QRS Nomenclature Examples

The QRS complex represents ventricular depolarisation, however, describing the QRS complex with these three letters is often an oversimplification. A QRS complex may not necessarily contain a Q wave, a R wave, or a S wave, and may contain more than one R wave.

The various deflections of the QRS wave can be notarised as follows:

             Q         q          R          r           S          s           R’         r’

Capitalisation of the letter indicates tall or deep waves, with small waves notarised with small letters.

Directionality and order also determines nomenclature, as below:

• Q wave – any initial negative deflection
• R wave – any positive deflection
• S wave – any negative deflection after a R wave
• R’ wave – any positive deflection that follows an S wave
• QS complex – if there is only a single negative deflection

Examples of various QRS complex morphologies and there nomenclatures below

 

References

•  Chan T, Brady W, Harrigan R, Ornato J, Rosen P (2005), ECG in Emergency Medicine and Acute Care, 1^st Edition, Elsevier Mosby.
• Hampton J (2003), The ECG in Practice, 4^thEdition, Churchill Livingstone.
• Surawicz B, Knilans T (2008), Chou’s Electrocardiography in Clinical Practice, 6^th Edition, Saunders Elsevier.
• Morris F, Edhouse J, Brady W, Camm J (2003), ABC of Clinical Electrocardiography, 1^st Edition, BMJ Books.